The invention relates to an electrically conductive layer comprising a uniform mixture of an electrical conductive component in the form of minute particles in an electrically nonconductive, curable polymer, and a method for the production of same.
The electrically conductive layer of the invention can be used to produce electrical resistors. In addition, the electrically conductive layer of the present invention may also be employed for screening or shielding purposes, for example, for grounding containers and the like.
Especially when used as an electrical resistor, there is a requirement that the temperature coefficient of the layer be both as small as possible and as constant as possible over a wide temperature range. The temperature coefficient is generally determined by dividing the change in the resistance value (based on the value at room temperature) by the resistance value at room temperature and the temperature difference. The temperature coefficient is especially important with resistance values having small tolerances. Therefore, a small and constant temperature is an especially important requirement for precision resistors.
In so-called organic thick layer technology, it is already known to produce layers for electrical resistors, whereby electrically conductive particles, such as soot, graphite, carbon fibers, silver, nickel, chromium or even metal alloys or metal oxides are imbedded in an organic, electrically insulating and simultaneously bonding polymer, such as, polyethylene or epoxy or phenolic resin. After curing, an electrical conductive matrix is formed, whereby the electrical conductivity of the layer is determined by, among other things, the fill concentration, the arrangement and electrical characteristics of the particles admixed in the polymer.
The temperature coefficient in a layer to which carbon particles are added is dependent on the temperature. The temperature coefficient of metal or metal oxide layers can also be influenced by the composition of the layer, whereby it is independent of the resistance value. In carbon layer resistors the attainable electrical conductivity is limited to low ohm values by the relatively poor conductivity of the admixed particles of graphite, soot, or carbon fibers, and the carbon layer resistors have a high negative temperature coefficient.
Especially when non-precious (and therefore affordable) metals are used for admixture, the electrical long-term stability often becomes questionable because of redox processes at the surface. One generally obtains resistors having a positive temperature coefficient.
In inorganic thick layer technology it is also known to produce so-called cermet resistors. Here, low-melting types of glass are employed as non-conductive and simultaneously bonding components. High quality, oxidation resistant metals, such as silver, platinum, ruthenium, palladium, etc, or their oxides are preferably used as the electrical conductive matrix. By mixing several pastes having differing electrical conductivities, one can alter the specific resistance and the temperature coefficient, whereby the conductivity of the resulting paste is dependent on the specific conductivity of the precious metal admixed with the glass frit and the mixture ratio thereof.
When soot or graphite is used as conductive particles in an electrically insulating polymer, there are several disadvantages. As previously mentioned, the conductivity of the layer is dependent on, among other things, the fill concentration of the particles, one must have on hand various masses with differing packing densities to obtain a broad spectrum of electrical resistance values. Differing packing densities (or concentrations), however, lead to varying rheological characteristics of the layer. In addition, the different packing densities cause different warping behavior during the curing of the layer. The varying surface tension from layer to layer, which results from the varying characteristics of the soot and the graphite, contributes to poor reproducibility of the resistance values from batch to batch, especially when a screening process is used in the production of the resistor layers.
To achieve different resistance values while the packing density (concentration) of the particles in the polymer remains constant, it is already known to form the particles representing the electrically conductive component as refractory, inorganic oxide materials, on whose outer surface is arranged a layer of a carbon-containing pyropolymer. The electrically conductive component comprises from 10-95% by weight based on the final composition of the mixture and the particle size is below 20 .mu.m. But different conductivities of such particles can only be obtained by varying the thickness of the layer of the pyrolytic carbon surrounding the individual refractory particles. The low range of conductivity values, which is necessary for high ohm resistance arrangements, can be obtained by greatly reducing the carbon-containing pyropolymer layer to a few single strata. The thus-attained high resistance values, however, are associated with an increasing degeneration of the behavior of the temperature coefficient. The explanation for this appears to reside in the relatively weakly defined continuity of the grain boundaries of the carbon layers. As the thickness of the layer decreases these contact points increase in significance. Because the stray resistance of the contact points with the grain boundaries is extremely sensitive to temperature, this condition is macroscopically expressed in a rapidly impaired temperature coefficient of the resistor as the carbon layer thickness decreases. Therefore, thicker layers of material with higher specific resistance values but the same resistance per area have lower resistor temperature coefficients.